The Parliamentary Information and Communications Technology department (PICT) will be responsible for upgrading on-site machines.
However, Stephen Pound, Labour MP for Ealing North questioned the move.
"What consideration [has] the House of Commons Commission... given to the installation of Windows 7 on computers on the House of Commons part of the Parliamentary estate; and for what reason [is] Windows Vista... being installed on such computers?," he asked Parliament.
In response, House of Commons Commission representative Nick Harvey said the machines need to be upgraded immediately as support for Windows 2000 expires in June 2010, but Windows 7 is still in the process of being tested by PICT.
"Given the timescales for the withdrawal of Windows 2000 support and the relatively recent availability of Windows 7 the move to Windows Vista for administrative staff was considered the best option given the circumstances," Harvey added.
"There is: a newer, better, less resource hungry, but fully supportable, workable and manageable solution is there," he said.
See also: Hackers use UK gov't PCs in a botnet